This invention relates to an impeller for centrifugal pumps, and more particularly to an improved solid impeller for a low specific speed pump having low flow and a high pump head.
It has been known in the art that a solid impeller is useful in a low specific speed pump to prevent lowering of pump efficiency which is caused by relative circulation of the treating liquid within the impeller flow passage. In FIGS. 1 and 2, a solid impeller 10 is shown, which comprises a disk-shaped body 14 having an inlet portion 12 at its center on one side thereof and a plurality of discharge passages 20 radially extending from the center portion to a peripheral region of the body. The discharge passage 20 has an inlet port 16 at one end and a discharge port 18 at its opposite end. Unlike the normal impeller, the solid impeller for a low specific speed pump has its impeller flow passage comprising a flow passage 20 for suppressing the relative circulation of the treating liquid to thereby prevent lowering of the pump efficiency.
The above radial discharge passages 20, however, cause the following disadvantages. For the radial discharge passage 20, it is required that a fluid inlet angle alpha defined by an inlet direction and a discharge direction of the treating liquid is normally set at a relatively large angle, for example, 90.degree.. The inlet port area S is set relatively small. Under the above circumstances, a variation of the actual fluid inlet angle of the treating liquid at the inlet port 16 is unavoidable. The variation of the fluid inlet angle of the treating liquid may cause an increase in a contraction effect. The increase of the contraction effect may cause suction inefficiency of the impeller discharge passage. This may result in a lowering of the pump efficiency and a generation of cavitation. Those problems are more serious when operating at a low specific speed.
To solve the above problem, another solid impeller has been proposed and is disclosed in the Japanese Utility Model Publication No. 57-45427. The other solid impeller is illustrated in FIGS. 3 and 4. A solid impeller 22 comprises a disk-shaped body 14 having an inlet portion 12 at its center portion on one side thereof and a plurality of discharge passages 20 radially extending from the center portion to a peripheral region of the body. The discharge passage 20 has an inlet port 16 at its one end and a discharge port 18 at its opposite end. Further, at the inlet port 16, an enlarged chamber 24 is provided to expand the diameter of the discharge passage 20. This is so that auxiliary inlet blades 26 are provided in the enlarged chamber 24. The blades 26 extend from a peripheral wall of the enlarged chamber 24 toward a center of the body to separate individual inlet ports 16. This is for a rectification of the fluid inlet of the treating liquid at the inlet port 16. The rectification effect of the fluid inlet may suppress both the variation of the fluid inlet angle of the treating liquid and the increase of the contraction effect caused by the variation of the fluid inlet angle. As a result, the suction of the discharge passage 20 is improved thereby lowering the pump efficiency. Cavitation may also be suppressed.
This solution, however, has other problems as described below. The other solid impeller has the enlarged chamber 24 having the auxiliary inlet blades 26 for improving the suction of the inlet port 16 of the discharge passage 20. Additional processes are required for preparing the plural auxiliary inlet blades 26, in particular welding of a back of the inlet blade 26. This requires complicated fabrication processes that may result in a high manufacturing cost of the solid impeller.
Alternatively, as illustrated in FIGS. 5 and 6, each of the discharge passages 20 may comprise a straight passage radially extending from the center of the body to the peripheral region thereof. The discharge passage 20 has the inlet port 16 and the discharge port 18, both of which are the same size in width and height.
The solid impeller 10 comprises the straight discharge passage 20. By having a straight discharge, relative circulation of the treating liquid in the discharge passage 20 may be suppressed. This may occur in a low specific speed pump. Also, a decrease in the pump efficiency which may be caused by the relative circulation may also be suppressed.
Notwithstanding, the above solid impeller having the straight discharge passage 20 has the following disadvantages. In the above solid impeller, the passage for the treating liquid comprises only the straight discharge passage 20. This is to suppress the relative circulation of the treating liquid in the discharge passage 20 and thereby any lowering of the pump efficiency is prevented. On the other hand, a disk friction loss due to the impeller itself or a lowering of the pump efficiency is unavoidable. The disk friction loss Pd is given by the following equation: EQU Pd=K.sub.1 r u.sup.3 D (D+5B.sub.2) (1)
where K.sub.1 is the coefficient, r is the specific weight of liquid (Kg/cm.sup.3), u is the peripheral speed (m/s), D is the outer diameter of impeller (m) and B.sub.2 =h.sub.2 =2t: the thickness of the impeller body at its peripheral part. The coefficient K.sub.1, the specific weight r, the peripheral speed u, and the outer diameter D are defined by the specific speed. The thickness B.sub.2 is optionally set to match the various conditions.
In the above conventional solid impeller, as illustrated in FIGS. 5 and 6, the discharge passage 20 is formed to have the same sizes both between w1 and w2 and between h1 and h2. This requires the thickness B.sub.2 to be set at a relatively large thickness. If the thickness B.sub.2 is set at a small value, the height h.sub.2, the height h.sub.1, and the width w.sub.1 are small thereby resulting in a decrease in suction. As a result, the disk friction loss (Pd) achieves a considerably large value. The friction loss is increased by increasing the outer diameter D of the disk or by lowering the specific speed.
Consequently, the conventional solid impeller may prevent the relative circulation loss of the treating liquid. However, it still may have a problem due to generating disk friction loss of the impeller body. Therefore, a substantial improvement in the pump efficiency is not achieved.
In the general view of the design of the impeller for the centrifugal pump, the specific speed n.sub.s =n Q.sup.0.5 /H.sup.0.75 is very important, where the flow is Q(m.sup.3), the pump head is H(m), and the rotation speed is n(r.p.m.). In the normal specific speed, for example, not less than 100, a shroud impeller 11 having a shroud 11a is available as illustrated in FIG. 7. In a lower specific speed than 100, an open impeller 21 comprising a radiation blade 21a is available as illustrated in FIG. 8. Alternatively, the solid impeller 10 comprising the disk-shaped body 14 including a plurality of passages 20 provided radially as illustrated in FIGS. 9 and 10 is also available.
In the shroud impeller 11 of FIG. 7, an outer diameter is made large to match the low specific speed. In this case, the friction loss or the disk friction loss due to the shroud 11a is rapidly increased and is proportional to five times the outer diameter. This results in a considerable lowering of the pump efficiency. From the above, it is found that the shroud impeller is unsuitable when using a low specific speed.
In the open impeller of FIG. 8, even if the outer diameter of the impeller is made large, almost no increase of the disk friction loss appears. This is due to elimination of the shroud. Notwithstanding, when having a lower specific speed, a circulation flow loss due to vortex flows 23 within the passage is generated. This results in a considerable lowering of the pump efficiency. From the above, it is found that an open impeller is not available when using very low specific speeds.
By contrast, the solid impeller 10 having the straight discharge passages 20 may prevent any generation of circulation loss over the entire range of the specific speed, particularly in very low specific speeds. For that reason, the solid impeller has been widely used, particularly for the low specific speed pump.
Such a solid impeller, however, has the following problems. In the solid impeller 10, an axial thrust T=A(P.sub.d -P.sub.s) is generated having a suction pressure P.sub.s (Kg/m.sup.2), a discharge pressure P.sub.d (Kg/m.sup.2), and a pressure area A (m.sup.2). The axial thrust (T) of the solid impeller is considerably large due to a large pressure area (A). The axial thrust (T) is increased by reduction of the low specific speed. To suppress the generation of the axial thrust, it is required to provide the impeller body 14 with an extra rear fixed orifice 14c and a balance hole 14d having a considerably large diameter. Such a solid impeller has a complicated structure and a heavy weight. This results in both an imbalance of the axial thrust and in a generation of a leakage loss 25. The fixed orifice 14c also has bearing problems. Therefore, the reliability of pump operations is decreased. The balance hole 14d also permits bypass of the treating liquid and therefore the pump efficiency is lowered. By contrast, the open impeller 21 having the small pressure area A, as illustrated in FIG. 8, may prevent both lowering of the reliability of the pump operation and lowering the pump efficiency.
In the solid impeller 10, the body 14 has a united structure. In this way, the disk friction loss is considerably large, although the shroud impeller 11 of FIG. 7 shows a larger disk friction loss. The amount of the friction loss of the solid impeller is further increased by the increase of the diameter of the impeller and the decrease of the specific speed. This results in a considerable decrease of the pump efficiency. The open impeller 21 having the radiation blades 21a is free from a lowering of the pump efficiency.
Under a low specific speed condition, the open impeller has the problem of a decrease in the pump efficiency due to the circulation loss. The solid impeller also has problems in lowering the reliability of both the pump operation and pump efficiency due to the axial thrust, leakage loss, and impeller friction loss.